Arbusers

Do you convert the odds to percentages and work out the difference in percentages?

For example, if you took $3 =33% about a selection and the selection started $2 = $50%.

So 50%-33% = beat SP by 17%.

If I understood you correctly, you ask about Starting Price at the Kick-Off of the event.

Every initially aired odds by the Asian compilers is de facto basic percentage probability for a given outcome, based upon let's say the expert opinion (or some hidden factors for some reason) of the compilers' staff minus the desired margin. Further, in the interval time, between initial publishment of those odds, till kick-off of the event, the opinions of the masses and the big money cause movement of the odds and eventually shaped the Closing Line.

In brief - Starting Price at the KO is the closing point of that time dependence function - initial expertise odds minus margin, influenced over some time till the event begins by the money flow factor. That's why the CL in my opinion is not the ultimate price to be beaten by you.

I am using the Betfair Starting Price as my closing line or SP. It is the correct way to measure things.

@Steve Johnston,
I'm afraid I can't be helpful to you.
Suppose you are participating in horse racing or similar, like greyhounds racing. In the past, I used to build models for every high liquidity and non-liquidity racing in a given country, based on some experience and research via the Internet and my own research and statistics processing. Never used to work with SP due to the reason, pinpointed in my previous post. Every price, triggered by my model was used as an entry or exit point, even CL if was acceptable.
Maybe you have to ask other experienced users which are using similar to yours' betting principles.